Pressure swing adsorption (PSA) is a well-known process for recovering light gases from mixtures which also contain heavier, more readily adsorbable components. The recovery of hydrogen from steam-methane reformate containing hydrogen, carbon oxides, and methane is a particularly well-suited application of the PSA process. Petroleum refinery gases containing hydrogen and hydrocarbons also are readily separated by the PSA process. The process is also useful for recovering helium from natural gas streams.
In a steam-methane reforming (SMR) process coupled to a PSA process for recovering hydrogen, a typical PSA feed from the SMR contains 70 vol % hydrogen, 25 vol % carbon dioxide, 4 vol % methane, and 1 vol % carbon monoxide. During the cyclic PSA process for hydrogen recovery, a reject gas stream containing up to 35 vol % hydrogen is withdrawn as adsorber vessel void space gas and/or purge effluent at low pressure, and this reject gas stream is typically used as reformer fuel. Because of this hydrogen loss, only about 80-85% of the hydrogen in a steam-methane reformate can be recovered by a conventional PSA system. It is desirable to increase overall hydrogen recovery by further treatment of the reject gas if such further treatment is cost effective.
One potential method to increase hydrogen recovery is to recycle a portion of the reject gas to the PSA feed. This can increase hydrogen recovery, but often is not cost effective because the low pressure reject gas which contains hydrogen as a minor component must be recompressed to a typical PSA feed pressure of 200-400 psig, although the feed pressure can range between 100 and 1000 psig. In addition, recycle reduces the net hydrogen concentration in the PSA feed, which reduces PSA performance if this concentration falls below about 70 vol %.
Various methods have been described in the art for using hydrogen-selective polymeric membranes to enrich PSA reject gas for recycle to increase hydrogen recovery. U.S. Pat. Nos. 4,229,188 and 4,238,204 describe the use of such hydrogen-permeable membranes for recycling a hydrogen-enriched gas to the feed of a multiple-bed PSA system. Membrane permeate which contains up to 97 vol % hydrogen is recompressed and recycled to the PSA to increase overall hydrogen recovery to 90%.
U.S. Pat. No. 4,836,833 discloses the operation of a two-stage multiple-bed PSA system for recovering hydrogen and carbon monoxide from steam-methane reformate. Carbon dioxide is removed from the feed gas in a parallel set of first stage adsorbers, and hydrogen is separated from carbon monoxide in a parallel set of second stage adsorbers. Reject gas from the second stage adsorbers is recompressed and passed through multiple polymeric membrane stages and the permeate, which is enriched in hydrogen and carbon dioxide, is recycled to the feed of the first stage adsorbers.
U.S. Pat. No. 4,783,203 discloses a hybrid PSA/membrane system for the recovery of a light component such as hydrogen from a mixture containing heavier components such as carbon monoxide. The PSA purge effluent is compressed and passed through a hydrogen-selective polymeric membrane module, and the hydrogen-lean nonpermeate gas is utilized as a displacement gas in the PSA system. Hydrogen-rich permeate gas optionally is used as a low pressure purge gas in the PSA system purge step.
These applications of polymeric membrane systems for improving PSA hydrogen recovery are characterized by a large pressure differential across the membrane which requires initial compression of the reject stream to provide a high pressure polymeric membrane feed (typically greater than 200 psig). Hydrogen selectively permeates through the membrane, so that recompression is required to recycle the hydrogen-rich permeate to the PSA system (typically 200 to 300 psig) for increased hydrogen recovery. These compression steps comprise a significant portion of the capital and operating cost of using a polymeric membrane system for improving PSA hydrogen recovery.
U.S. Pat. No. 5,104,425 discloses a composite semipermeable membrane comprising microporous adsorptive material supported by a porous substrate, and teaches the use of this membrane for separation of gas mixtures including hydrogen-hydrocarbon mixtures. This membrane differs from conventional polymeric membranes in that hydrocarbon impurities preferentially diffuse through the membrane and the hydrogen-rich product is withdrawn as a nonpermeate stream at a pressure slightly below the feed pressure.
Improved methods for hydrogen recovery will be needed as the expected demand for hydrogen increases in the petroleum refining, transportation, and related industries. In particular, it is desirable to increase hydrogen recovery when operating PSA systems on steam-methane reformate or refinery waste gases containing hydrogen. The present invention, which utilizes an adsorbent membrane separator integrated with a PSA system and steam-methane reformer as disclosed and defined in the following specification and claims, addresses this need for more efficient methods for the recovery and purification of hydrogen.